This invention relates to a method of controlling the operation of a two-speed motor compressor that may selectively be operated at a predetermined high speed or at a predetermined low speed and that is used to drive a hermetic refrigerant compressor, and more particularly, to a novel method of controlling the operation of such two-speed electrical motor, so as to attain an overall motor compressor efficiency at low speed operation which is equal to or greater than the overall motor compressor efficiency at high speed operation.
Even before energy conservation became a matter of general, popular interest, the art had been concerned about minimizing the amount of energy consumed by heating, ventilating, and air conditioning equipment. Such concern has motivated scrutiny of equipment designs and application practices and procedures.
It has long been standard practice to "size" air conditioning equipment utilized in light commercial applications so that it can have ample cooling capacity for peak load conditions. To provide comfort under part load conditions, which are present during a large portion of the time, provisions are then made so that a given system can operate at a cooling capacity level somewhat lower than that used during peak load conditions. With the refrigerant compressor used in the air conditioning system, such provisions have included, inter alia, cylinder unloading mechanisms, suction valve lifting mechanism, and hot gas by-pass valves. While most provisions utilized for unloading refrigerant compressors have satisfied the comfort aspects of light commercial applications and some have reduced gross energy consumption, their characteristics have been to provide lower efficiency when unloaded than when loaded.
Recently, it has been proposed to affect capacity control of a hermetic refrigerant compressor by driving the compressor by a two-speed electric motor. U.S. Pat. No. 3,584,980, issued June 15, 1971 to Richard E. Cawley and Charles B. Ellis discloses such a hermetic two-speed motor compressor system wherein the motor drives the compressor at a high speed during peak load conditions and drives the compressor at a lower speed during part load conditions.
The novel method of my present invention is directed to a two-speed, motor compressor system of the type disclosed in U.S. Pat. No. 3,584,980. By the use of my method, the operation of the two-speed motor compressor system can be controlled such that the overall coefficient of performance (efficiency) of the system at the lower speed, i.e., during part load conditions, is equal to or greater than the coefficient of performance (efficiency) of the system at the higher speed, during peak load conditions. Basically my novel method comprises the steps of imposing a first torque load on the two-speed electric motor during peak load operation and under a given set of evaporating and condensing conditions and then imposing a second torque load on the electric motor during part load operation and under the same set of evaporating and condensing conditions such that the ratio of the second torque load to the first torque load is equal to or less than the ratio of the motor efficiency at part load operation to the motor efficiency at peak load operation.
Accordingly, an object of the present invention is to provide a novel method of controlling the operation of a hermetic two-speed, electric motor driven, refrigerant compressor so as to attain an overall compressor efficiency at low speed operation which is equal to or greater than the overall motor compressor efficiency at high speed operation.
This and other objects and advantages of the present invention will become apparent from the following description of the preferred embodiment of the invention described in connection with the accompanying drawing.